Single-side or double-side copper-clad laminates are usually being manufactured by a process in which an electrodeposited or rolled copper foil which has undergone a surface treatment for adhesion is superposed on prepregs obtained by impregnating a base material such as a woven glass fiber fabric or the like with ah epoxy resin or the like as a matrix resin and drying the impregnated base material, or such prepregs are sandwiched between two such copper foils, and the resulting assembly is lamination molded. Multilayered printed circuit boards having an inner-layer printed circuit are manufactured by a process in which the above copper-clad laminate is combined with an inner-layer board having formed thereon a printed circuit for inner layer use and further with prepregs and either a copper foil or a single-side copper-clad laminate and the resulting assembly is subjected to lamination molding to give a multilayered board both sides of which are constituted by a copper foil, and this multilayered board is subjected to processing steps such as steps for forming through-holes, outer-layer printed circuits, etc.
The most conventional method of the surface treatment for bonding of copper foils is to impart surface roughness by an electrolytic technique. The surfaces of the copper foils thus surface-treated normally have a roughness height of about 3 to 9 .mu.m, but such a roughness is not preferable particularly from the standpoint of high-frequency characteristics. In addition, there has been a drawback that the adhesive strength of the roughened surfaces is reduced as the roughness height is lowered.
On the other hand, copper foils, particularly rolled copper foils, which have a finely roughened surface layer formed by treatment of the surface with an aqueous solution of an oxidizing agent and is composed of a copper oxide of a black to brown color are known. Since these copper foils have a small surface roughness height, they are suitable for high-frequency uses and other applications. However, they have a drawback that the finely roughened surface layer composed of a black to brown copper oxide is brittle and tend to easily dissolve in acidic aqueous solutions such as hydrochloric acid, sulfuric acid, and the like. Because of this, there is a problem that in the case where multilayered boards produced by lamination molding are processed to form through-holes therein and then subjected to through-hole plating, electroless plating, or subsequent electroplating, the acidic solution penetrates into the copper oxide layer from exposed parts of the layer in the through-hole walls and attacks the copper oxide layer to cause so-called "harrow" or "pink ring", which may impair insulating and other properties, resulting in final printed circuit boards with poor reliability. In addition, there is another drawback that great care should be taken in transporting or handling the surface-roughened copper foils, because of the brittleness of the copper-oxide surface layer.